1. Field of the Invention
This invention relates to a photoelectric conversion device for converting light into electric signal and in particular to a photoelectric conversion device having a high sensitivity, utilizing the charge multiplication effect. Such photoelectric conversion device includes e.g. a photocell, a one-dimensional image sensor, a two-dimensional image sensor, an image pick-up tube, etc.
2. Description of the Related Arts
Heretofore, as photoelectric conversion elements, whose principal component is an amorphous semiconductor, there are known a photocell, a one-dimensional image sensor (e.g. JP-A- No. 52-144992), a two-dimensional image sensor combining a solid state drive circuit with an amorphous semiconductor (e.g. JP-B- No. 59-26154), a photoconductive image pick-up tube (e.g. JP-A- No. 49-24619), etc. Some of these photoconversion devices adopt a blocking type structure having a junction characteristic of preventing charge injection from the signal electrodes to the photoconductive layer and some others adopt a structure, by which charge is injected from one or both of the electrodes, a so-called injection type structure.
In an injection type element, since it is inherently possible to take-out charge carriers which are larger in number than the incident photons, a high sensitivity with a gain greater than 1 can be realized. In order to increase the sensitivity of the photoconversion element stated above, an imaging device has been proposed, in which a reading-out circuit and a photoconductive layer having e.g. phototransistor characteristics are overlaid upon each other (JP-A- No. 61-222383).
In order to achieve a similar object, a method utilizing an electrostatic induction type transistor as a device having a multiplication effect in its photoelectric converting portion itself has been proposed (JP-A- No. 57-21876) (IEEE Transactions on Electron Devices, Vol. ED 22, (1975) pages 185-197). There has been proposed also a method, by which a p.sup.+ .pi. p n n.sup.+ structure is formed using an amorphous semiconductor, whose principal component is Si containing hydrogen and/or halogen (e.g. fluorine, chlorine, etc.), which structure is similar to that formed using crystalline Si, in which avalanche multiplication takes place in the depletion layer of its p-n junction portion in order to amplify signals. On the other hand, in the case where the blocking type structure having a characteristic of preventing charge injection from the exterior of the photoconductive layer is adopted, since only the portion of the incident light, which is converted into electric charge within the photoconductive layer, generates a signal current, the gain of the photoelectric conversion is always smaller than 1. However, it has been proposed by the inventors of this application that even a device of blocking type structure can have a photoelectric conversion efficiency greater than 1, if a method is adopted, by which a blocking type structure is formed by an amorphous semiconductor layer, whose principal component is Se and in which avalanche multiplication is made to occur in order to amplify signals (U.S. patent application Ser. No. 69156).
As described above, when the injection type structure is adopted for a photoelectric conversion device such as a photocell, a one-dimensional image sensor, a photoconductive layer piled-up type solid state photosensitive device, etc., since it is inherently possible to take-out charge carriers larger in number than the incident photons, a high sensitivity with a gain greater than 1 can be realized. However, by this method, by which a part of electric charge is injected in the interior of the sensor, the photoresponse is significantly deteriorated.
Further, in the case of the electrostatic induction type transistor, it was difficult to have uniform multiplication factors at a same value for different pixels, because an amplifying portion was integrated in each of the pixels.
On the other hand, in the example in which an amorphous semiconductor is used, since it is possible to form a homogeneous layer at a relatively low temperature and in addition the layer has a high resistivity, advantages can be obtained that no complicated pixel separation process as for crystalline Si is needed to realize a high resolution characteristic. However, for a photosensitive element, to which the avalanche multiplication phenomena in amorphous semiconductor are applied, there still remain several problematical points.
That is, by the method by which a p.sup.+ .pi. p n n.sup.+ structure identical to that adopted for an avalanche diode made of a crystalline semiconductor is formed using amorphous Si in order to amplify signals, a signal light is projected through the p.sup.+ region in the .pi. region, where it is absorbed and converted into electric charge, which is in turn led to the p-n junction portion, and the avalanche multiplication takes place in the depletion layer of the p-n junction portion. In order to cause the avalanche multiplication, it is necessary that electric charge travels over a distance longer than a certain value. The present inventors test-fabricated the structure stated above using amorphous Si, and confirmed that since localized states existing in the forbidden band were more numerous for amorphous Si than for crystalline Si, the depletion layer in the p-n junction portion did not satisfactorily extend, resulting in insufficient avalanche multiplication effect. Further, it was recognized that when the operating temperature exceeded room temperature, dark current was increased, and it was not possible to apply an electric field thereon, which was so high that a sufficient avalanche multiplication effect could be obtained. These results indicate that there was a problem that no satisfactorily high amplification factor could be obtained only by forming an avalanche diode structure similar to that in crystalline Si by using amorphous Si.
Furthermore, in the case of the avalanche multiplication method using amorphous Se having a blocking contact structure (i.e., structure which blocks carrier injunction from the associated electrode), although a large multiplication factor and a good photoresponse can be obtained, because of restrictions due to the material itself, e.g. in a high temperature environment over 80.degree. C., there is a fear that the layer is altered during use and in particular there is a problem that element characteristics are unsatisfactory at high temperature operation.